1. Field of the Invention
This invention relates to an imaging technique and system for detecting the propagation of mechanical waves within a body structure of a patient.
2. Background
Certain medical conditions, such as diagnosis of myocardial ischemia, are often difficult to establish in their early stages when treatment is most effective. Patients suffering from myocardial ischemia may present to an emergency room or acute care facility with typical cardiac symptoms such as chest pain, described as tightness, pressure, or squeezing, but some patients may have other symptoms such as arm or chin pain, nausea, sweating, or abdominal pain. Standard techniques such as electrocardiogram often provide inconclusive findings regarding ischemia, and sometimes may even be unable to identify situations in which ischemia has progressed to cell damage and myocardial infarction (MI). More reliable techniques are available for diagnosing infarction relative to its predecessor, ischemia. For example, a blood test to measure the creatine kinase-MB (CK-MB) enzyme level is used for detection of myocardial cell damage. Other useful serum markers include troponin I, and to a lesser extent, myoglobin. However, the blood levels of these compounds may take several hours to rise, so that diagnosis of MI may be delayed. Reliance on blood tests alone may result in a significant loss of time when early aggressive therapy is warranted.
Less invasive diagnostic techniques have become available through the observation of mechanical properties of tissue via imaging techniques. Such evaluation of the function of the heart, cardiovascular tissue, or other body structures is based on the mechanical interpretation of the movement of the these structures, such as, for example, the active contractions and passive relaxation of the myocardium. For example, U.S. Pat. No. 5,542,298 to Sarvazyan et al. describes a technique and apparatus for determining the physical state parameters of a medium by generating oscillations in a reference medium.
Using current imaging techniques, the evaluation of the heart function is mainly based on a single mechanical interpretation of myocardial deformation. In order to quantify these deformations, several techniques have been introduced, such as tissue doppler imaging (Sutherland et al., 1995), strain rate imaging (Heimdal et al., 1998), and myocardial elastography (Konofagou et al., 2002) in the field of ultrasound imaging, and cardiac tagging (Declerck et al., 2000) in the field of magnetic resonance imaging. By use of these techniques, the deformations of the myocardium are quantified over a complete cardiac cycle in order to provide some information on the myocardial viability.
Low frequency mechanical vibrations in the heart were shown by Kanai et al. (Kanai and Koiwa, 2001; Kanai et al., 1993), in human patients. They developed a new ultrasound system to demonstrate that several pulsive mechanical vibrations were obtained around end-systole and end-diastole in the frequency range of 25 to 100 Hz. U.S. Pat. No. 5,840,028 to Chubachi, Kania and Koiwa describes their ultrasonic imaging equipment for the measurement of small vibrations in myocardial tissues. The apparatus and technique described in the '028 patent have several drawbacks, including requiring the use of complicated ultrasound systems that may not be clinically applicable.
Accordingly, there is a need in the art for a diagnostic tool for determining characteristics of body structures which avoids the drawbacks of the prior art.